Conventionally, in an aircraft of the kind which has a crew or passenger cabin which is pressurized to enable the aircraft to fly at high altitudes without providing a local oxygen supply to each passenger and crew member e.g. via a breathing mask, an emergency oxygen supply is available for use in the event of a demand or an increased demand for oxygen or oxygen enriched gas, for example in the event that the cabin becomes depressurized. Such emergency oxygen supply may be provided from compressed gas storage containers and/or by combining two or more chemicals which undergo a reaction which produces oxygen gas (e.g. chlorate candles), and would be supplied to passengers and crew by individual breathing masks.
By providing such an emergency supply of oxygen gas, time is available for a pilot to reduce flying height to an altitude where the crew and passengers may again breath atmospheric gases. However such an emergency supply is, only available for a short period of time.
It is usual practice particularly in the case of civilian aircraft, for flying routes taken by aircraft to be arranged such that in the event of an emergency, such as cabin decompression, the aircraft is within 30 minutes or so flying time from land. Thus for safety's sake, the route taken by an aircraft may not be the shortest and most economical route.
Moreover, even though an aircraft may be within 30 minutes flying time from land, often a suitable landing ground is not available for landing the aircraft within this flying range e.g. the nearest land may be hostile territory, and where an aircraft is constrained to fly at relatively low altitude, typically less than 10,000 feet, during low altitude flight over some land masses, the aircraft may encounter terrain at a height at or greater than 10,000 feet, or adverse weather conditions.
It is known more particularly for military aircraft, for an emergency oxygen supply apparatus to be provided which is capable of supplying oxygen indefinitely. Such oxygen supply apparatus may be of the molecular sieve bed type which when operated adsorbs oxygen gas from a gas supply thus to provide a gas which is sufficiently oxygen enriched for breathing at higher altitudes, or may be of the gas permeable membrane type which allows oxygen or non-oxygen gas to permeate through a membrane thus to enrich a supply gas for breathing, and it has also been proposed to supply oxygen using a ceramic oxygen generator to separate oxygen from a gas supply.
Such oxygen supply apparatus tend to work most efficiently particularly in terms of start-up time, where of relatively small capacity. To use such technology in a civilian aircraft with a large number of passengers, would thus require a plurality of such oxygen supply apparatus. For passenger aircraft now being proposed which will be capable of carrying 700 passengers or more, it will be appreciated that a substantial number of oxygen supply apparatus would be required to ensure an adequate oxygen supply for all passengers in the event of an emergency which results in increased demand for oxygen. Additionally, because such oxygen supply apparatus are not readily able to produce oxygen instantly, conventionally it would still be necessary to carry e.g. compressed oxygen which can be used in the event of an emergency decompression, until such oxygen supply apparatus come on line. All this adds to the weight of the aircraft, which is undesirable for economic reasons.